feat: notification digests via SMTP

Adds a periodic inbox-summary email per opted-in user. SMTP is
configured via appsettings (Smtp section); the digest is built from
existing analytics (health score, top senders, recommendations) and
sent by a new hourly DigestWorker, mirroring the GmailSyncWorker
pattern. Frequency and send hour are configurable (Digest section).

Backend: IEmailSender (SmtpEmailSender, MailKit), IDigestService,
DigestWorker, new User.DigestEnabled/LastDigestSentUtc fields +
migration, SettingsController for the per-user toggle and a
send-now test endpoint, all scoped to the authenticated UserId.

Frontend: SettingsApi + a bell/no-bell toggle button in the topbar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/styles.css

@@ -412,3 +412,5 @@ input, select { background: var(--panel-2); border: 1px solid #2c3550; color: va
 .saved-search-save-btn:hover { color: var(--text); border-color: var(--accent); }
 
 .email-row--focused { outline: 1px solid var(--accent); outline-offset: -1px; }
+
+.digest-toggle { font-size: 16px; padding: 4px 8px; }